The present invention pertains to optical power meters and, more particularly, to apparatus and methods for automatically adapting and calibrating such meters to a plurality of optical cables, systems and applications.
Fiber optic cables are used in telephony, cable television, local area networks, industry and medicine. In communications, fiber optic cables carry pulsed modulated optical signals, originating from lasers or light emitting diodes, for communicating voice and data signals. In industry, fiber optic sensors transmit over fiber optic cables signals whose intensity and wavelength indicate the nature of a sensed parameter. Fiber optic cables are used in medicine to transmit, e.g., high powered laser beams for cutting and vaporizing tissue.
The proper utilization of fiber optic systems requires precise instruments for measuring optical power and testing the integrity of optical circuits. The design of such instruments is complicated by the many applications of, and types of facilities using, fiber optic systems. Depending upon the application and facility, a multitude of different operating powers, signal wavelengths and structural and mechanical configurations are possible. For example, a typical facility utilizing optical communication systems may comprise an integrated services digital network (ISDN) for telephonic communications and a local area network (LAN) for internal data communications. Since the optical, structural and mechanical characteristics of each of these systems are different, an optical power meter designed for testing one system often cannot be used for testing the other system. This problem is particularly acute with respect to the connectors used to terminate optical circuits at communications interfaces. A multitude of such connectors presently are being used within the communications industry, and, although various standards have been proposed, none has been widely adopted.
Various attempts have been made to make optical power meters more adaptable. For example, some optical power meters contain both a source of optical power and a sensor for sensing optical power. A source of optical power is necessary to test the integrity of optical circuits, and measure the attenuation of optical power caused by such circuits, when a source is not active on the circuit. A number of connecting adaptors also are sold with, or for, optical power meters to enable the meter's connection to some of the many different connectors used to terminate fiber optic circuits at communication interfaces. Means also have been provided for manually entering into the meter, if known, the wavelength of the optical source being measured to enhance the accuracy of optical power measurements.
Although these attempts have made such instruments somewhat more adaptable, they also have made them more complicated to use. The technician must repeatedly calibrate and recalibrate the instrument, depending upon the source of optical power, even if the source is that supplied by the instrument itself. If the instrument has adaptors for a particular type of terminal connector, a different application requires that the technician remove the adaptors and reconnect new adaptors, and this operation generally requires recalibration. If the technician is working in a facility in which two types of terminal connectors are employed, he or she is required to repeatedly remove and attach adaptors, and recalibrate the instrument, or use two instruments.
Several attempts to increase the adaptability of optical power meters have employed separate source and sensing modules. Depending upon the application, the technician can select and insert the appropriate source and sensing modules. There is no guarantee, however, that the particular modules selected will be compatible or that the instrument will be calibrated properly for these modules. The technician also must be concerned with selecting the correct modules and an appropriate connecting adaptor for the configuration. Whenever the configuration is changed, moreover, the instrument must be recalibrated, and whenever a different type of installation is tested, the modules or adaptors must be changed. All of these modifications complicate the testing and monitoring process, and substantially increase the likelihood of errors.
Errors also have been generated inherently in such plug-in systems as a result of temperature gradients. For example, a temperature gradient between the source module and sensing module, a condition which is likely if these modules are stored or installed separately, will generate errors, even if the instrument has been calibrated for the particular modules being used.